In the fields of iron and steel products, semiconductor products, display products, and the like, surface films having thicknesses of a few tens to a few hundreds of nanometers may be controlling factors of product characteristics. In the field of iron and steel products, surface thin films having thicknesses of a few tens to a few hundreds of nanometers may be controlling factors for press formability of the products. In recent years, many of steel sheets to be used for automobiles and household electrical appliances are subjected to galvanization from the viewpoint of high corrosion resistance. However, in the case where a hard forming part is produced by press-forming of the plated steel sheet, there is a problem in that press-cracking easily occurs at a site, which undergoes severe forming, in the steel sheet. In a known method for improving the press formability of the galvanized steel sheet by using a high-viscosity lubricating oil during pressing, there is a problem in that variations occur in downstream steps, e.g., a conversion treatment and painting, unless a degreasing process is enhanced.
A method, in which a coating taking a lubricating action is formed on a surface of a plating layer, is known as a method for decreasing the above-described fear in the downstream steps. For example, Japanese Unexamined Patent Application Publication Nos. 53-60332 (page 1), 2-190483 (page 1) and 2004-3004 (page 2) disclose a technology for improving the weldability or the formability by subjecting a surface of the galvanized steel sheet to an electric field treatment, a dip treatment, an oxidation treatment after coating, or a heat treatment so as to form an oxide film mainly containing a zinc oxide. Japanese Unexamined Patent Application Publication No. 4-88196 (page 1) discloses a technology for improving the press formability and the conversion treatment performance by dipping a galvanized steel sheet in an aqueous solution containing 5 to 60 g/L of sodium phosphate and exhibiting a pH of 2 to 6, performing an electric field treatment, or applying the above-described aqueous solution so as to form an oxide film mainly containing a phosphorous oxide on a surface of the plated steel sheet. Japanese Unexamined Patent Application Publication No. 3-191093 (page 1) discloses a technology for improving the press formability and the conversion treatment performance by performing an electric field treatment, a dip treatment, a coating treatment, an oxidation treatment after coating, or a heat treatment so as to form Ni oxide on a surface of a galvanized steel sheet.
The most direct manner for evaluating the press formability of the plated steel sheet is, for example, to perform pressing in practice with a full-size test machine imitating a mold to be used in a practical automobile part production and evaluate on the basis of an occurrence of cracking or wrinkling due to the pressing. However, this testing method requires a full-size test piece, large facilities, and efforts. Therefore, a sliding property testing method has been put into practical use as a method for evaluating the sliding property which is an important factor of the press formability. Examples of such a sliding test disclosed include a method in which one surface or both surfaces of a test piece is pressed against a die, the test piece is pulled out, the friction coefficient is determined from the pull-out resistance of the die and the test piece at this time, and the press formability is evaluated on the basis of the friction coefficient (refer to Japanese Unexamined Patent Application Publication No. 2004-3004 (page 2), for example), and a method in which the press formability is evaluated by an evaluation method based on the contact sliding of a metal body over a plated steel sheet (refer to Japanese Unexamined Patent Application Publication No. 2003-136151 (page 2), for example).
On the other hand, as is clear from the above-described known technologies, the press formability is controlled by the thickness of the lubricating film formed as the surface layer of the plated steel sheet. Therefore, the press formability can also be evaluated on the basis of the film thickness. In particular, in the case where an oxygen-containing film (oxide film) taking a lubricating action is formed on a surface of the plating layer, the sliding property is changed significantly depending on the oxide film thickness. Consequently, the sliding property can be evaluated by measuring the thickness of the oxide film, and it is possible to use as a simple alternative index of the press formability.
The known technologies for measuring the oxide film thickness are as described below:    (1) A method in which the information in the depth direction is measured by combining a surface analysis technique, e.g., Auger electron spectroscopy or X-ray photoelectron spectroscopy, and ion etching;    (2) A method in which a sample showing a cross section is prepared and observed with a transmission electron microscope from a film thickness direction; and    (3) An optical technique, e.g., ellipsometry, by using an interference effect of light in a thin film.